Spiropyran derivatives are most well-known as typical organic compounds which reversibly colors or decolorizes upon exposure to the energy of light or heat. Examples and properties of these derivatives are collectively described, for example, in G. H. Brown, Photochromism (John Wiley & Sons, Inc., 1971).
However, attempts to introduce conventional spiropyran derivatives into use, for example, as recording media encounter the following problems.
(1) Since the colored form (or the colorless form) present in a solution or high polymer binder is low in stability to light or heat, the system immediately returns to colorless (colored). (2) When repeatedly exposed to light and heat for coloration and decolorization (recording and erasure), the exposure to light gives rise to a side reaction, which decomposes or deteriorates the spiropyran derivative. Thus, the derivative is not fully resistant to repetitions. (3) Although the spiropyran derivative for use as a photochromic medium is usually dispersed in a high polymer substance, the derivative dissolves out from the high polymer substance, or separates out therefrom through phase separation since the derivative generally has low compatibility with the high polymer substance.
An object of the present invention is to provide a novel spiropyran compound capable of easily giving a high polymer spiropyran compound which is usable free of the drawbacks of the conventional photochromic materials.